Hi Mona,
With enough skids under the pallet, the common construction of 3/4" plywood battened with 3/4" pine can often suffice for moderately heavy objects, and 2x4 lumber battens can work for very heavy objects if they are not too large. This becomes problematic for larger dimensions though, as the open structure of even a heavy pallet can sag under its own weight once it reaches a certain size, providing little structural support for the object.
If rigid support is needed, one alternative option you might consider is a heavy-duty lumber battens combined with a torsion box design for the load-bearing pallet. There are many approaches to a torsion box, but one example would be to replace the pine battens with 2x4s or 2x6s, flip them onto their narrow edges to make a kind of skirt around the perimeter of the pallet, add more 2x material to span the interior of this skirt in both directions at regular intervals (forming a grid), and skin the underside of the pallet as well as the top. This creates a fully enclosed box with a lumber grid inside. Joining each section of lumber securely to the plywood sandwich provides a rigidity otherwise unavailable in these materials. The tighter the grid inside the box, the thinner the top & bottom skins can be without sacrificing rigidity. For something this heavy I would try 3/4" plywood for the skins, and also consider a second layer of ply on top of the torsion box if the object rests on any narrow feet that can puncture the plywood. The caveat to an approach like this is the additional weight of the lumber. Full disclosure: I have not yet had the necessity of resorting to this approach in a crating context, but it is a relatively quick and inexpensive method to test compared to steel fabrication.

Chris